It is generally preferable that a dispersion is in a state where particles are stably dispersed and remain floating or suspended in liquid without aggregating together, and attaining a desired dispersion state is an important target of research and development in current industries. Adjustment and control of particle dispersion state is under study in various fields such as inorganic particles (e.g. magnetic materials), cosmetics, pigments, and foods. If a desired dispersion state is stabilized and, for example, the stabilized dispersion state is preserved even when the concentration is raised, improved efficiency and productivity of production processes can be attained by using a concentrated solution without performance deterioration. Further, in the field of coloring, colorants which can give deeply-, uniformly-, and brilliantly-colored dyed products can be manufactured, and such colorants will have increased commercial values. Properties of particles are generally better when the particle diameter is smaller, and there is a strong demand for stable dispersions containing particles having a diameter of sub-microns or of nanometers.
According to an inkjet recording method, high speed recording can be performed with a high freedom degree of imaging pattern and a low noise at the time of recording. Further, image recording can be performed in a short period of time and at low costs. Still further, the inkjet recording method has advantages such that color recording can be readily performed. Therefore, recently the inkjet recording method is rapidly spreading and further developing. As a recording liquid for the method, hitherto a dye ink, in which a water-soluble dye is dissolved in an aqueous medium, has been widely used. However, the dye ink is poor in water resistance and weather resistance of the resultant printed article. Therefore, studies of the dye ink have been made to improve such disadvantages.
A pigment ink is ordinarily obtained by dispersing a water-insoluble pigment in an aqueous medium. Hitherto, it is general to use a method which includes, adding a pigment together with one or plurality of dispersing agents such as various kinds of surfactants or water-soluble polymers to an aqueous solvent, and pulverizing them using a dispersing device such as a sand mill, a bead mill, or a ball mill, to make the diameter of the pigment particle fine. In contrast, dispersions formed by the build-up method of generating pigments and the like in liquid phase are also under development. For example, a method for preparing a pigment dispersion liquid by dissolving an organic pigment together with a polymer dispersant or a polymer compound as a dispersant in an aprotic organic solvent in the presence of alkali, and then mixing the resultant solution with water, is disclosed (see JP-A-2004-43776 (“JP-A” means unexamined published Japanese patent application)). In addition, polymer compounds and the like for use in the build-up method are studied (see JP-A-2003-26972, JP-A-2003-113341, JP-A-2006-342316, and JP-A-2007-119586), and dispersion stability of particles is also reviewed (see e.g., Gijutsu-joho Kyokai (Technical Information Institute Co., Ltd.) Ed., “Saishin ‘Ganryo-bunsan’ Jitsumu Know-how·Jireishu (Newest ‘Pigment Dispersion’, Practical Know-how and Examples)” published in October 2005, pp. 208-211). However, the dispersion stability is not still satisfactory, and there is a need for a further improving development.
In aqueous inkjet-ink printing, such a phenomenon is known that printing of a figure requiring a large amount of ink such as full-color photograph causes curling of the paper after ink ejection, and there is a demand for prevention of curling. The reason for curling is considered because hydrogen bonds of cellulose, a component of a paper which serves as the support, are cleaved by penetration of the ink solvent into the paper and then re-bind in random states when dried. As a method for preventing curling of paper, a method of adding a hydrophobic organic solvent having a large Log P value to the ink is proposed (see Hirotaka Iijima, Kenichi Okubo, and Kunitsuna Sasaki, “Konica Minolta Technology Report” Vol. 4 (2007) and JP-A-2006-192586).